This invention pertains generally to the production, maintenance, and storage of slush hydrogen, and particularly to the production of slush hydrogen by magnetic refrigeration.
Slush hydrogen is an equilibrium mixture of solid and liquid hydrogen that exists on the solid-liquid line of hydrogen, e.g., at conditions of 13.8 K and 7.033 KPa (1.02 psia). Slush hydrogen has a greater density and lower enthalpy than hydrogen in its liquid state, and these properties make slush hydrogen an attractive alternative to liquid hydrogen in applications such as for a propellant in aerospace vehicles. The greater density of slush hydrogen as compared to liquid hydrogen allows for volume savings in the propellant, resulting in a smaller tank and a corresponding increase in payload to gross take-off weight ratio and a reduction of airframe size and weight. The lower enthalpy results in greater heat sink capacity for a given hydrogen mass to absorb engine and aerodynamic heat loads.
Several methods of producing slush hydrogen have been suggested in the prior art. These include vacuum pumping, helium gas refrigerator cooling, Joule-Thomson cooling of a helium-hydrogen gas mixture, and helium gas-injection cooling.
The vacuum pumping method may be of several varieties. One variation of the vacuum pumping method is straight vacuum pumping, where evaporation of some of the liquid from a storage tank provides the refrigeration to reduce the temperature of the remaining liquid to the freezing point and then, by continued evaporation, to produce mixtures of solid and liquid or to completely freeze the remaining hydrogen. A second variation of the vacuum pumping method is semiflow vacuum pumping. In the vacuum pumping of the semiflow variety, saturated liquid hydrogen, at one atmosphere, supplied from a transport vessel is first expanded through a Joule-Thomson valve into an evacuated tank held at a pressure slightly above the triple point, and then is vacuum pumped in that tank until the desired slush quality is obtained. Vacuum pumping methods can lose up to 15% of the liquid hydrogen during slush hydrogen production and require a subatmospheric tank pressure, which is conducive to atmospheric leaks and thus the formation of explosive mixtures in the tank.
A second method of producing slush hydrogen is by means of a helium gas refrigerator, which can be performed in either a batch or a flow process. This method has an advantage over the vacuum pumping techniques in that no hydrogen is lost during production. However, this method requires more costly and complex equipment installation.
A third method of producing slush hydrogen is Joule-Thomson cooling of a helium-hydrogen gas mixture.
In this process, the gas mixture is precooled by a regenerative heat exchanger and then passed through a Joule-Thomson expansion valve to achieve the desired quality slush hydrogen as the condensed phase with a helium-hydrogen vapor phase. The slush is withdrawn from the production vessel, and the helium-hydrogen vapor is recirculated. Hydrogen is added to the process stream to replace the hydrogen withdrawn as slush. Joule-Thomson cooling can be operated at or above atmospheric pressure and therefore eliminates the need for evacuable storage vessels and vacuum pumps. The main disadvantage of this method is that it accomplishes essentially the same results as vacuum pumping, but with added capital and operating costs.
In another method, helium gas is injected into a bath of liquid hydrogen. The temperature of the liquid hydrogen is lowered as a result of the vaporization of a portion of the liquid hydrogen into the bubbles of non-condensing helium as they rise through the liquid hydrogen. The cooling achieved by this process is dependent upon the amount of liquid which is vaporized and carried from the bath by the flow of the injected helium gas. The large helium volume required for this process causes the helium injection method to have a very high operating cost.
The above methods of producing slush hydrogen typically operate at approximately 10% of the Carnot efficiency. These systems require large volumes and weights for pumps, compressors, and other equipment; for example, in a 1 Kw at 12 K unit, the helium gas refrigerator occupies about 35 m3 in size and weighs about 1800 kg. Some methods leave contaminants in the slush hydrogen and/or hydrogen exhaust gas. For example, the vacuum pumping technique results in oil vapor in the hydrogen exhaust. Such contaminants are a major source of fouling in the Joule-Thomson expansion valves.
In accordance with the present invention, a slush hydrogen production device includes an insulated tank which encloses a hydrogen slushifier magnetic refrigerator, i.e. a magnetic refrigerator which is used to cool and solidify hydrogen. The hydrogen slushifier magnetic refrigerator provides cooling at or about 12 K while rejecting heat at or about 21 K. The vacuum insulated tank of hydrogen is divided into two regions by the vacuum enclosure around the magnetic refrigerator. The first or upper compartment contains an ullage space of gaseous hydrogen and primarily liquid hydrogen which is used as a heat sink by the hydrogen slushifier magnetic refrigerator, with the slushifier refrigerator being located within the evacuated partition. The second or lower region of the hydrogen vessel contains the slush hydrogen. Liquid hydrogen migrates between the regions to supply make-up hydrogen for that converted to solid and removed as slush. Stratification will generally occur in the lower region with liquid hyrdrogen at 20.2 K near the top of the lower region and near 13.8 K liquid hydrogen further down in the lower region, and slush hydrogen near the bottom of the tank.
During operation, the hydrogen slushifier magnetic refrigerator requires electric power and two flows of liquid hydrogen, and produces gaseous hydrogen and solid hydrogen. One flow of liquid hydrogen is used to remove heat from the slushifier magnetic refrigerator. The flow leaves the upper compartment and passes through a pipe to the magnetic refrigerator slushifier. The hydrogen is vaporized, absorbing heat from the refrigerator, and is returned to the upper compartment as a gas, via another pipe. The gaseous hydrogen leaves the hydrogen vessel at near 21 K through a pipe exiting the vessel. The second flow of liquid hydrogen is a thermal load at near but above 13.8 K which enters the refrigerator from the lower compartment via an insulated pipe. The hydrogen slushifier magnetic refrigerator solidifies the second flow of liquid hydrogen at 13.8 K, and the solidified hydrogen flows and falls back into the lower compartment, increasing the solid concentration in the hydrogen as it mixes with the liquid hydrogen. When the solid concentration becomes near 50%, the slush hydrogen is pumped to an independent storage tank at the rate that it is formed, and is replaced by liquid hydrogen which flows from the top region to the lower region of the hydrogen container. The feed liquid hydrogen is gradually cooled from near 20.2 K to near 13.8 K by direct contact heat exchange in the stratification layer of liquid hydrogen.
The hydrogen slushifier magnetic refrigerator utilizes the magnetocaloric effect by which certain magnetic materials increase in temperature when placed in a magnetic field and decrease in temperature when removed from the field. The slushifier refrigerator includes a wheel of paramagnetic or ferromagnetic material which rotates to bring all points on the wheel periodically into and out of a strong magnetic field produced by superconducting magnets. When entering the magnetic field at a high temperature heat transfer region, the magnetic material in the wheel increases in temperature and rejects heat by the vaporization of liquid hydrogen flowing from the upper compartment; the vaporized hydrogen is then vented back to the upper compartment. When a portion of the wheel leaves the magnetic field, it decreases in temperature and passes to a low temperature heat transfer region where liquid hydrogen at near 13.8 K from the lower compartment freezes directly upon the wheel. The solidified hydrogen is then removed from the wheel and allowed to fall to the bottom of the lower compartment, forming slush hydrogen.
The superconducting magnets are preferably formed as solenoids disposed so that the magnetic wheel rotates through the bores of the magnets. The magnetic field is varied about the periphery of the magnetic wheel by selective placement of the magnets and by altering the number of windings from magnet to magnet. The magnetic wheel is driven to rotate through the variable magnetic field by an electric motor located outside of the vacuum insulated tank, the wheel being supported along its periphery by bearings. The solid hydrogen is removed from the wheel in the low temperature heat transfer region by a rotary scraper that is also driven by the electric motor, the solid hydrogen falling to the bottom of the lower. compartment in the vacuum insulted tank to form slush.
A second, smaller magnetic refrigerator is preferably employed to keep its own magnets and the magnets of the hydrogen slushifier magnetic refrigerator well below the superconducting transition temperature. For initial start-up, a gaseous helium expansion precooler is used to cool the magnets of the hydrogen slushifier refrigerator and the magnets of the second refrigerator to the appropriate temperature range. Once the magnets are appropriately cooled, the magnets can be energized and the precooler can be shut off.
The use of magnetic refrigeration to produce slush hydrogen offers high reliability, ease of use, and ease of maintenance because a minimum number of relatively slow moving parts are required. The magnetic refrigerator slushifier of the present invention operates near 40% of the Carnot-efficiency and provides a stand-alone device for in-situ production and maintenance of hydrogen slush that is of small volume and weight; for example, a 1 kW at 12 kg unit occupies approximately 1 m3 and has a mass of 350 kg. All external connections of the slushifier magnetic may be at atmospheric pressure, there is no need for large quantities of gaseous helium, and no contaminants are left in the slush hydrogen product or gaseous hydrogen exhaust.
Further objects, features, and advantages of the invention will be apparent from the following detailed description when taken in conjunction with the accompanying drawings.